1. Field of the Invention
The present invention relates to a technique useful for photolithography in the production of a semiconductor device and, more particularly, to a photoresist material and a negative resist composition, which are suited for lithography using far ultraviolet light of not more than 220 nm, particularly ArF excimer laser beam, as light for exposure, and a process for forming a pattern using the same.
2. Description of the Related Art
In the field of the production of various electronic devices represented by semiconductor device, which requires fine working in the order of half micron, the requirement for higher density and more increased integration of the device has been enhanced. Therefore, the requirement for photolithography for formation of a fine pattern has becomes strict, more and more.
A means for making an attempt to obtain a fine pattern includes, for example, an attempt to shorten the wavelength of light for exposure used in the formation of a pattern using a photoresist. Therefore, in the mass production process of 256 Mbit (working size is not more than 0.25 .mu.m), the employment of a KrF excimer laser having a shorter wavelength (wavelength: 248 nm) as a light source has been studied in place of i-line (wavelength: 365 nm).
In the production of DRAM having an integration degree of 1 Gbit (working size is not more than 0.18 .mu.m), which requires a more fine working technique, a light source having a shorter wavelength is required. At present, the employment of photolithography using an ArF excimer laser (193 nm) has been studied [Donald C. Hoffer et al., Journal of Photopolymer Science and Technology, Vol.9 (No.3), pages 387-397 (1996)].
It is necessary that lithography employing excimer laser satisfies an improvement in cost performance of laser because life of a gas as a raw material of laser oscillation is short and a laser apparatus itself is expensive. Therefore, the requirement for higher sensitization is also enhanced, in addition to higher resolution corresponding to an attempt to obtain a fine working size.
As a photoresist having high sensitivity, for example, a chemical amplification type resist employing a photo-acid generating agent as a photosensitive material is well known. A feature of the chemical amplification type resist is that protonic acid generated from a photo-acid generating agent as a constituent component after light exposure causes an acid catalytic reaction to a base resin of the resist during a heat treatment after light exposure. In such way, the photoresist has attained a markedly higher sensitization than that of a conventional resist having a reaction efficacy (reaction per one photon) of smaller than 1. Typical examples of the chemical amplification resist includes a resist comprising a combination of triphenylsulfonium hexafluoroarsenate and poly(p-tert-butoxycarbonyloxy-.alpha.-methylstyrene) described in Japanese Patent Kokai Publication No. 27660/1990. As a negative resist, a resist comprising a combination of polyvinylphenol and a melamine derivative of L. E. Bogan et al. is described in Proceeding of SPIE. Vo.1086, pages 34-47 (1989).
As the resist for g-line, i-line or KrF excimer laser, for example, a resin having a benzene ring, such as novolak, polyvinylphenol or the like have been used. However, the resin having a benzene ring exhibits extremely strong light absorption to light having a wavelength of not more than 220 nm, such as ArF excimer laser beam or the like. Therefore, when using these resists in ArF excimer laser lithography, almost all of light for exposure is absorbed on the surface of a thin film. As a result, light for exposure does not penetrate into a substrate so that a fine resist pattern can not be formed. Accordingly, the resin, which has been used in g-line, i-line or KrF excimer laser, can not be applied to photolithography using a light having a short wavelength of not more than 220 nm.
On the other hand, dry etching resistance, which is essential to the resist for production of the semiconductor, has been obtained from the benzene ring in the resin in the case of g-line, i-line or KrF excimer laser. Therefore, a photoresist material, which does not contain the benzene ring and has etching resistance and is transparent to a wavelength of not more than 220 nm, is required as a resist for exposure to ArF excimer laser.
A positive resist, which has transparency to ArF excimer laser beam (193 nm) and has dry etching resistance, has been studied intensively during the past few years. Regarding these resists, a resin having an alicyclic group is used as the base resin. Typical examples thereof include copolymer having an adamantyl methacrylate unit [Journal of Photopolymer Science and Technology], Vol.5 (No.3), pages 439-446 (1992), copolymer having an isobornyl methacrylate unit [R. D. Allen et al., Journal of Photopolymer Science and Technology, Vol.8 (No.4), pages 623-636 (1995), and Vol.9 (No.3), pages 387-397, pages 465-474 (1996)], copolymer having a carboxylated tricyclodecylmethyl methacrylate [Maeda et al., Proceeding of SPIE, Vol.2724, pages 377-398 (1996)] and the like.
However, a methacrylate derivative having an alicyclic group used in the former resin does not have a polar group having substrate adhesion (e.g. carboxyl group, hydroxyl group, etc.). Therefore, a homopolymer of a monomer having an alicyclic group has strong hydrophobicity and poor adhesion with a substrate to be worked (e.g. silicon substrate, etc.) so that it is difficult to form a uniform coated film with good reproductivity. Furthermore, the homopolymer does not have an adamantane-containing residue having dry etching resistance, an isobonyl-containing residue or a residue capable of affording a difference in solubility before and after exposure in a menthyl-containing residue unit and, therefore, a pattern can not be formed by exposure. Accordingly, the former resin can not be employed as the resin component of the resist until the resin is copolymerized with a comonomer capable of affording a difference in solubility, such as t-butyl methacrylate, tetrahydro methacrylate, etc. or a comonomer having substrate adhesion, such as methacrylic acid, etc. However, it is necessary that the content of the comonomer is about 50% by mol and the dry etching resistance of a comonomer unit is drastically low and, therefore, the dry etching resisting effect due to an alicyclic group is drastically lowered, resulting in poor practicality.
As described above, a positive photoresist having both dry etching resistance and high definition has already been known in a photoresist for light having a short wavelength of not more than 220 nm, which is represented by ArF excimer laser lithography. However, a negative photoresist having both dry etching resistance and high definition has never been found out. Therefore, a negative resist which can be employed in lithography using light of not more than 220 nm, such as ArF excimer laser, as light for exposure, that is, a negative resist having both dry etching resistance, definition and substrate adhesion (fine pattern is not easily peeled off from a substrate) for lithography using light of not more than 220 nm, such as ArF excimer laser, as light for exposure is earnestly desired.
Japanese Patent Kokai Publication No. 233455/1991 discloses a negative photoresist composition, Japanese Patent Kokai Publication No. 291259/1992 discloses a resist composition, Japanese Patent Kokai Publication No. 045879/1993 discloses a resist composition and a process for forming a pattern using the same, and Japanese Patent Kokai Publication No. 248625/1995 discloses a radiation-sensitive resin composition, respectively. Furthermore, Japanese Patent Kokai Publication Nos. 259626/1996, 15846/1997, 221519/1997 and 221526/1997 to the present inventors each discloses the related invention.